1. Field of the Invention
This invention relates to polydiorganosiloxane compositions which are stable in the absence of moisture but which are curable with heat and moisture to yield elastomeric materials having a dry, non-tacky surface. This invention further relates to a method of producing said elastomeric materials.
2. Description of the Prior Art
In the early days of commercial silicone rubber one of the problems that afflicted the peroxide-curable organopolysiloxane compositions was the tacky surface that was obtained when the compositions were cured in the presence of oxygen. A solution to this problem, albeit a tedious solution, was to cure the compositions in the absence of oxygen. A better solution was to use a dichlorobenzoly peroxide to cure the composition as taught by Youngs in U.S. Pat. No. 2,732,966.
This problem of surface tack still exists with the peroxide-cured organopolysiloxane compositions of today that are based on low-viscosity polydiorganosiloxanes. The use of a dichlorobenzoyl peroxide does not eliminate the surface tack of peroxide-curred low-viscosity polydiorganosiloxanes.
Acetoxysilanes have been mixed with certain organosilicon compositions for various reasons for a long time. It was therefore surprising to find that the surface tack of a heat-cured composition comprising a low-viscosity polydiorganosiloxane can be eliminated simply by mixing, with the uncured composition, an alkenyltriacetoxysilane.
It is known to mix an organic peroxide with room-temperature-curable silicone compositions which comprise acyloxysilanes. For example, Ceyzeriat, in U.S. Pat. No.3,264,258 discloses a vulcanizable, moisture-free liquid composition comprising a linear, hydroxy-terminated diorganopolysiloxane that is crosslinked both by an organic peroxide and by an organotriacyloxysilane. The compositions of Ceyzeriat cure to elastomeric materials when exposed to moisture, in the well-known fashion, and may be optionally further cured by heating to confer additional thermal stability to the moisture-cured composition.
Silanes have also been mixed with heat-curable compositions that are based on high-viscosity polydiorganosiloxanes. Simpson. in U.S. Pat. No. 3,341,489 discloses a heat-curable composition comprising 100 parts of an organopolysiloxane polymer having a viscosity of at least 100,000 centipoise at 25.degree. C., 10 to 300 parts of a filler, and 0.1 to 10 parts of an olefinically unsaturated organosilicon material. The compositions of Simpson can further comprise from 0.1 to 8 parts of an organic peroxide. The olefinically unsaturated organosilicon material in Simpson's compositions can be any of a number of materials including acetoxysilanes, but alkenyltriacetoxysilanes are neither taught specifically nor preferred. Przybyla, in U.S. application Ser. No. 600,692, of July 31, 1975, entitled "Heat-curable Silicone Elastomers Having Unprimed Adhesion" and assigned to the assignee of this application, discloses the mixing of a room-temperature-curable silicone elastomer stock having silicon-bonded acetoxy radicals with an organic-peroxide-curable organopolysiloxane to obtain a heat-curable elastomeric composition having unprimed adhesion. The room-temperature-curable silicon elastomer stock can be preformed and admixed with the organic-peroxide-curable organopolysiloxane or the components of said elastomeric stock comprising a hydroxy-endblocked polyorganosiloxane fluid and an acetoxysilane can be added to said organopolysiloxane. Stevenson, in U.S. Pat. No. 3,769,253 discloses a primer composition for heat-curable silicone rubber comprising a silicone polymer solution, an organic-peroxide curing catalyst and a silane of the formula R.sup.1.sub.x Si(OOCR.sup.2).sub.4-x in which R.sup.1 is an alkenyl radical having from 1 to 10 carbon atoms, R.sup.2 is an alkyl group having from 1 to 5 carbon atoms and x is an integer of from 1 to 2. Exemplary of the suitable compound is vinyltriacetoxysilane. The primer solutions of Stevenson are applied to a substrate and thereafter dried, preferably prior to the application of the silicone rubber that is to be bonded to the primed substrate. The assembly of substrate, primer composition and convertible silicone rubber is then heated under pressure to cure and bond the silicone rubber to the primed substrate.
The disclosures of Simpson and Przybyla and Stevenson are directed to compositions based on high-viscosity polyorganosiloxanes. Since the teachings of Youngs, supra, offer a solution to the problem of surface tack in peroxide-curable compositions based on high-viscosity polyorganosiloxanes but fail when applied to similar low-viscosity polydiorganosiloxanes, the teachings of Simpson and/or Przybyla and/or Stevenson would not be expected to offer a solution of the problem of surface tack in peroxide-cured low-viscosity polydiorganosiloxane compositions.